1. Field of the Invention
The present invention relates to a power steering system wherein power assistance derived therefrom will be modulated according to a vehicle operating condition such as, for example, vehicle speed.
2. Discussion of Background
In maneuvering a steering wheel of an automotive vehicle, it is recognized that power assistance at high vehicle speeds may desirably be less than at low speeds to allow for stabilization of the steering wheel at high speeds.
A power steering system with the above-mentioned steering characteristics is known and includes a reaction device which in accordance with fluid pressure supplied thereto provides resistance against relative rotation between valve elements of a servo-valve. The servo-valve controls fluid flow to and from a fluid motor according to the relative rotation between the valve elements, which, in turn, generates the power assistance. The fluid pressure supplied to the reaction device is controlled by a magnetic control valve at a low level when high power assistance is necessary at low vehicle speeds, and on the contrary, at a high level when less power assistance is required at high vehicle speeds.
Conventionally, the magnetic control valve includes a valve body which has a bore, an inlet port connected to the reaction device and is also supplied with fluid flow of a constant rate, and an outlet port connected to a reservoir. A spool valve is slidably mounted in the bore so as to vary the area of a variable orifice which is disposed between the inlet and outlet ports. A spring is provided which urges the spool valve in a direction to decrease the orifice area. A solenoid is mounted on the valve body and moves the spool by excitation thereof against the spring to increase the orifice area at low vehicle speeds or when the vehicle is stopped.
However, in the event where electric current supplied to the solenoid becomes null due to, for instance, problems occurring in the current supply circuit, the spool valve is urged to a position where the orifice area is a minimum. Accordingly, the fluid pressure supplied to the reaction device is always maintained at a maximum so as to result in a lack of power assistance at low vehicle speeds or when the vehicle is stopped.
Moreover, if excess electric current is continuously supplied to the solenoid because of problems in operation of a controller, the spool valve is overshifted to a position where the variable orifice opens wide. Accordingly, the fluid pressure supplied to the reaction device is kept low, resulting in excess power assistance and loss stabilization of the steering wheel at high vehicle speeds.